Image forming apparatus employing electrophotography are severely sought to be made higher in speed and higher in reliability. For example, the apparatus have begun to be used also for printing of high-definition images such as graphic designs, and further for light-duty printing (uses for the print-on-demand (POD) that enables printing in a great variety and a small volume, where documents can be edited of course, also copied and bound as well by means of a personal computer), which is required to be more reliable.
Further, as transfer materials to be used, a great variety of sheets of paper are recently used, such as reclaimed paper with a large surface unevenness and coated paper with a smooth surface. In order to cope with surface properties of such transfer materials, a fixing assembly is preferably used which affords a wide fixing nip like that of a soft roller or belt roller type. However, making the fixing nip wide enlarges the area of contact between a toner and a fixing roller to tend to cause what is called high-temperature offset in which a fused toner sticks to the fixing roller. Since the image forming apparatus are also used in bookbinding and the like as the uses for POD, there is also a strong desire for double-sided print. That is, a copying machine is desired which can reproduce images in a high quality on various transfer materials and on their both sides.
To meet such demand, it is particularly important to improve the toner in its fixing performance.
The storage elastic modulus (G′) and loss elastic modulus (G″) that are of dynamic viscoelasticity characteristics of the toner and the loss tangent tan δ (G″/G′) that is defined as the ratio of the former to the latter are known as one of physical properties which govern the fixing performance of the toner.
In PTL 1, in order to keep a toner from causing high-temperature offset and keep a transfer material from winding around a fixing roller, it is proposed to specify the shape of a fixing nip zone and also control the toner to have a tan δ at temperature 120° C. of from 1.7 to 5.0. However, to cope with more various image forming apparatus and transfer materials, it is insufficient to only control viscoelasticity characteristics at such a one-point temperature of 120° C.
In PTL 2, a technique is also proposed in which the ratio of G′ and value of tan δ of a toner are specified within a plurality of temperature ranges so as to improve the toner in its fixing performance and high-temperature offset resistance. However, the toner disclosed in PTL 2 has so high an absolute value for the tan δ that the toner is soft, and hence, to cope with more various image forming apparatus and transfer materials, there has been room for improvement.
In PTL 3, a technique is still also proposed in which, in a toner making use of as a binder resin a resin formed by mixture of a crystalline polyester and a non-crystalline polyester, the toner is controlled to have tan δ and G′ at temperature 120° C. so as to be improved in a balance between fixing performance and high-temperature offset. However, since a crystalline polyester and a non-crystalline polyester are used in mixture, a difference in degree to which the crystalline polyester is compatibilized may come about because of a difference in heat history especially where images are formed on both sides of a transfer material having surface unevenness, to cause non-uniform fixing between images on the surface and images on the back.
That is, in the above toners, to cope with more various image forming apparatus and transfer materials, there still has remained room for improvement.